US 7134970 B2 Abstract A putter-head (1) has its center of mass (15) spaced p mm behind its impact-face (8) at a height h_{c }mm above the head-bottom (9), a loft angle α_{12 }at height 12 mm above the head-bottom (9), a moment of inertia/kg-mm^{2 }about the vertical axis through the center of mass (15), a mass M kg and a radius of gyration of K mm about the heel-toe axis (2–10) of the head through the center of mass (15), where p/l is not more than 0.18, h_{c }is less than [12−p×sin(α_{12})]. The ratio d_{2}/K is less than 1.0, d_{2 }mm being the vertical offset above the heel-toe axis (2–10) of the axis of attachment of the putter-shaft (3) to the putter-head (1); the attachment-axis of the shaft may be spaced by no more than the shaft-radius from the center of mass. The impact-face (36) may have an upper flat section (38) that merges smoothly into a cylindrical lower section (39), and the head (30) may be constructed with a high-density part (32;40) that extend lengthwise of the heel-toe axis and is either bonded to the underside of a lower-density part (31), or forms both an upstanding front flange (43) and a rear body-section (41) of larger mass than, and spaced from, the flange (43).
Claims(24) 1. A putter-head having a principal heel-toe axis, a bottom, a single attachment point for attachment of a putter-shaft to the putter-head, an impact-face defining a strike-area for putting contact with a golf ball, the strike-area having a lower extremity and an upper extremity, and wherein:
(a) the putter-head has a center of mass located at a distance p millimeters behind the impact-face;
(b) the putter-head has a moment of inertia/kilogrammes-millimeters^{2 }about a vertical axis through the center of mass;
(c) the putter-head has a radius of gyration of K millimeters about the principal heel-toe axis;
(d) the single point of attachment of the putter-shaft to the puller-head is offset vertically from the principal heel-toe axis by a distance d^{2 }millimeters;
(e) the ratio p/l is not more than 0.18;
(f) the ratio d_{2}/K is less than 2.0;
(g) the strike-area of the impact-face has a loft that increases upwardly from the lower extremity of the strike-area to the upper extremity of the strike-area; and
(h) the loft of the strike-area is not more than +10 degrees and not less than −15 degrees.
2. The putter-head according to
12−p×sin(α_{12}) where α_{12 }is the loft angle of the impact-face at a height of 12 millimeters above the bottom of the putter-head.
3. The putter-head according to
4. The putter-head according to
5. The putter-head according to
6. The putter-head according to
7. The putter-head according to
8. The putter-head according to
9. The putter-head according to
10. A putter-head having a principal heel-toe axis, a bottom, a single attachment point for attachment of a putter-shaft to the putter-head, an impact-face defining a strike-area for putting contact with a golf ball, the strike-area having a lower extremity and an upper extremity, wherein:
(a) the putter-head has a center of mass located at a distance p millimeters behind the impact-face;
(b) the putter-head has a moment of inertia/kilogrames-millimeters^{2 }about a vertical axis through the center of mass;
(c) the putter-head has a radius of gyration of K millimeters about the principal heel-toe axis;
(d) the single point of attachment of the putter-shaft to the putter-head is offset vertically from the principal heel-toe axis by a distance d_{2 }millimeters;
(e) the ratio p/l is not more than 0.18;
(f) the ratio d_{2}/K is less than 2.0;
(g) the strike-area of the impact-face has a loft that increases upwardly from the lower extremity of the strike-area to the upper extremity of the strike-area;
(h) the loft of the strike-area is not more than +10 degrees and not less than −15 degrees; and
(i) a mass of M kilogrammes, wherein a function LR defined by:
is greater than 0.95 when:
where α_{5 }is the loft angle at height 5 millimeters and G is defined by:
11. The putter-head according to
where α_{17 }is the loft angle of the impact-face at height 17 millimeters above the bottom of the putter-head.
12. A putter-head having a principal heel-toe axis, a bottom, a single attachment point for attachment of a putter-shaft to the putter-head, an impact-face defining a strike-area for putting contact with a golf ball, the strike-area having a lower extremity and an upper extremity, and wherein:
(a) the putter-head has a center of mass located at a distance p millimeters behind the impact-face;
(b) the putter-head has a moment of inertia/kilogrammes-millimeters^{2 }about a vertical axis through the center of mass;
(c) the putter-head has a radius of gyration of K millimeters about the principal heel-toe axis;
(d) the single point of attachment of the putter-shaft to the putter-head is offset vertically from the principal heel-toe axis by a distance d_{2 }millimeters;
(e) the ratio p/l is not more than 0.18;
(f) the ratio d_{2}/K is less than 2.0;
(g) the strike-area of the impact-face has a loft that increases unwardly from the lower extremity of the strike-area to the upper extremity of the strike-area;
(h) the loft of the strike-area is not more than +10 degrees and not less than −15 degrees; and
(i) a mass of M kilogrammes, wherein LR is greater than 0.98 when:
where α_{5 }is the loft angle of the impact-face at a height 5 millimeters above the bottom of the putter-head.
13. The putter-head according to
where α_{17 }is the loft angle of the impact-face at a height of 17 millimeters above the base of the putter-head.
14. A putter-head having a principal heel-toe axis, a bottom, a single attachment point for attachment of a putter-shaft to the putter-head, an impact-face defining a strike-area for putting contact with a golf ball, the strike-area having a lower extremity and an upper extremity, and wherein:
(a) the putter-head has a center of mass located at a distance p millimeters behind the impact-face;
(b) the putter-head has a moment of inertia/kilogrammes-millimeters^{2 }about a vertical axis through the center of mass;
(c) the putter-head has a radius of gyration of K millimeters about the principal heel-toe axis;
(d) the single point of attachment of the putter-shaft to the putter-head is offset vertically from the principal heel-toe axis by a distance d_{2 }millimeters;
(e) the ratio p/l is not more than 0.18;
(f) the ratio d_{2}/K is less than 2.0;
(g) the strike-area of the impact-face has a loft that increases unwardly from the lower extremity of the strike-area to the upper extremity of the strike-area;
(h) the loft of the strike-area is not more than +10 degrees and not less than −15 degrees; and
(i) a function S is defined by:
where α_{i},is the loft angle of the impact-face at impact height h_{i }and his defined by:
h=h_{i}−h_{c}−p×sin(α_{i}) and h has value greater than 2 for all values of h_{i }between 5 and 17 millimeters.
15. The putter-head according to
16. A putter comprising a putter-head having a single shaft-attachment point, and a putter-shaft attached to the putter-head at the single shaft-attachment point of the putter-head, wherein the putter-head has a principal heel-toe axis, a bottom, an impact-face defining a strike-area for putting contact with a golf ball, the strike-area having a lower extremity and an upper extremity, and wherein:
(a) the putter-head has a center of mass located at a distance p millimeters behind the impact-face;
(b) the putter-head has a moment of inertia/kilogrammes-millimeters^{2 }about a vertical axis through the center of mass;
(c) the putter-head has a radius of gyration of K millimeters about the principal heel-toe axis;
(d) the single point of attachment of the putter-shaft to the putter-head is offset vertically from the principal heel-toe axis by a distance d_{2 }millimeters;
(e) the ratio p/l is not more than 0.18;
(f) the ratio d_{2}/K is less than 2.0;
(g) the strike-area of the impact-face has a loft that increases upwardly from the lower extremity of the strike-area to the upper extremity of the strike-area;
(h) the loft of the strike-area is not more than +10 degrees and not less than −15 degrees; and
(i) the putter-head has a rotational compliance relative to the putter-shaft of at least 0.3 degrees per Newton-meter for rotation about the principal heel-toe axis of the putter-head.
17. The putter according to
12−p×sin(α _{12}) where α_{12 }is the loft angle of the impact-face at a height of 12 millimeters above the bottom of the putter-head.
18. The putter according to
19. The putter according to
20. The putter-head according to
21. The putter according to
22. The putter according to
23. The putter according to
24. The putter according to
Description This application is a continuation-in-part of U.S. patent application Ser. No. 10/488,152 filed with an effective filing date of Sep. 2, 2002 now U.S. Pat. No. 6,951,517 which is national stage completion of PCT/GB02/03995 filed Sep. 2, 2002 which claims priority from British Application Serial No. 0210581.5 filed May 9, 2002, British Application Serial No. 0209060.3 filed Apr. 20, 2002, British Application Serial No. 0205962.4 filed Mar. 14, 2002, British Application Serial No. 0130838.6 filed Dec. 22, 2001, and British Application Serial No. 0121261.2 filed Sep. 1, 2001. This invention relates to putter-heads and is also concerned with putters including them. In putters, it is desirable to arrange that the putter-head behaves like a free body at impact and to control various parameters of the putter-head such as its principal moments of inertia, the position of its center of mass and the impact face shape in order to improve spin and velocity characteristics imparted on a ball at impact. These improvements comprise greater imparted topspin or reduced backspin and a reduction in the variation of putt length as a function of impact height. According to one aspect of the present invention there is provided a putter-head having a principal heel-toe axis, a bottom, a single attachment point for attachment of a putter-shaft to the putter-head, an impact-face defining a strike-area for putting contact with a golf ball, the strike-area having a lower extremity and an upper extremity, and wherein (a) the putter-head has a center of mass located at a distance p millimeters behind the impact-face; (b) the putter-head has a moment of inertia/kilogrammes-millimeters^{2 }about a vertical axis through the center of mass; (c) the putter-head has a radius of gyration of K millimeters about the principal heel-toe axis; (d) the single point of attachment of the putter-shaft to the putter-head is offset vertically from the principal heel-toe axis by a distance d_{2 }millimeters; (e) the ratio p/l is not more than 0.18; (f) the ratio d_{2}/K is less than 2.0; (g) the strike-area of the impact-face has a loft that increases upwardly from the lower extremity of the strike-area to the upper extremity of the strike-area; and (h) the loft of the strike-area is not more than +10 degrees and not less than −15 degrees According to another aspect of the invention a putter comprises a putter-head having a single shaft-attachment point, and a putter-shaft attached to the putter-head at the single shaft-attachment point of the putter-head, wherein the putter-head has a principal heel-toe axis, a bottom, an impact-face defining a strike-area for putting contact with a golf ball, the strike-area having a lower extremity and an upper extremity, and wherein: (a) the putter-head has a center of mass located at a distance p millimeters behind the impact-face; (b) the putter-head has a moment of inertia/kilogrammes-millimeters^{2 }about a vertical axis through the center of mass; (c) the putter-head has a radius of gyration of K millimeters about the principal heel-toe axis; (d) the single point of attachment of the putter-shaft to the putter-head is offset vertically from the principal heel-toe axis by a distance d_{2 }millimeters; (e) the ratio p/l is not more than 0.18; (f) the ratio d_{2}/K is less than 2.0; (g) the strike-area of the impact-face has a loft that increases upwardly from the lower extremity of the strike-area to the upper extremity of the strike-area; (h) the loft of the strike-area is not more than +10 degrees and not less than −15 degrees; and (i) the putter-head has a rotational compliance relative to the putter-shaft of at least 0.3 degrees per Newton-meter for rotation about the principal heel-toe axis of the putter-head. The combination of a compliant shaft coupling arrangement and a variable loft impact face in a putter according to the invention has been found to endow it with advantageous characteristics that are not achieved with known forms of putter. In particular, the de-coupling of the shaft stiffness at impact significantly reduces the effective height of the horizontal rotation axis parallel to the impact face (the principal heel-toe axis) and the effective moment of inertia about this axis and, combined with variable putter face loft, generally reduces backspin, provides a mechanism whereby variations in putt length as a function of impact height are reduced and allows implementations where backspin is eliminated over the useful part of the impact face. The location of attachment of the putter-shaft to the putter-head has been found to have a significant effect on putting characteristics of the head. In particular, ratios d_{1}/K and d_{2}/K, relating respectively the horizontal offset d_{1 }millimeters of the attachment from the said heel-toe axis, and its vertical offset d_{2 }millimeters above that axis, to the radius of gyration K, are relevant. Either or, desirably, both may have a value less than 1.0, or more preferably, less than 0.33. The horizontal offset d_{1 }may in particular have a value that is less than the radius r (in millimeters) of the putter-shaft, and may indeed be zero, and the vertical offset d_{2 }may be negative. More especially, the spacing of the attachment from the center of mass may with advantage be no more than K millimeters, or even r millimeters. Furthermore, the impact-face of the putter-head of the invention may have upper and lower sections that are contiguous with, and merge smoothly into one another, the upper section being a flat surface and the lower section having the form of the surface of a cylinder that has its axis parallel to the heel-toe axis of the putter head. In another form, the impact-face of the putter-head of the invention may have two or more flat sections, delineated along the horizontal, with upper sections having higher loft than lower sections. The putter-head may comprise two parts a first of which is of high-density material and the second of which is of a lower-density material, the two parts being bonded together with the first part under the second part or located low down within it. Alternatively, the putter-head may be of metal or high-density composite having an upstanding front flange and a rear body-portion spaced from the flange, wherein the rear body-portion spaced from the flange, wherein the rear body-portion is of larger mass than the flange and extends beyond it in either direction lengthwise of the heel-toe axis. Putter-heads in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Referring to In practice there may be departure from the somewhat strictly-rectangular configuration shown for the base 4, to incorporate stylistic features, angled surfaces and rounded edges. In order to conform to the Rules of Golf, the putter head of As shown most clearly by The base 4 is of metal or other high-density material and provides a high proportion of the overall mass of the putter-head 1. The bumper 5, in contrast, is of low-density material so as to have a very low mass, and is preferably of a material with high modulus-to-density ratio such as a high strength plastics or a fibre-reinforced composite. It is preferably significantly harder and more rigid than a golf ball (i.e. harder than 70 Shore D), and is rigidly bonded or otherwise attached to the base 4. In this manner, the base 4 and bumper 5 form a substantially single rigid body, with negligible flexibility in the mechanical interface between them. The dimensions of the recess 11 are chosen to optimise the location of the center of mass of the head 1 low down and rearwardly of the flange 6. The removal of material from the body 4 by way of the recess 11, reduces the mass of the head 1 but also shifts the center of mass downwardly and backwardly depending on the depth and horizontal extent to which the recess 11 is taken. The shift downwardly and backwardly is accompanied by re-distribution of the resultant mass outwardly towards the heel 2 and toe 10. This helps to reduce rotation of the head 1 about its central vertical axis, and therefore to improve putt accuracy, in circumstances where impact with the ball is laterally offset from this axis. The bumper 5 is designed for adequate strength but minimum weight, since its weight has significant influence on performance. It provides a very lightweight, rigid interface between the impact-face 8 and the base 4 and experiences negligible deformation during putting impact; impact deformation that does occur is limited primarily to the golf ball and/or the impact-face 8. Although the bumper 5, and the impact-face 8 along with it, might be extended so as to be of comparable length to the base 4, this would add superfluous weight where it is not wanted. The element 7 is of a material having specific hardness and/or resilience and/or ball traction properties; typically it is of a different material from the bumper 5, but instead of being separate from the bumper 5 as in the present case, may be formed as part of it. The bumper 5 spaces the impact-face 8 of the element 7 several millimeters forwardly of the base 4 to ensure that there is a large separation between the face 8 and the center of mass of the putter-head 1. The front flange 6 is the highest part of the bumper 5 and is higher than any part of the base 4, and the vertical and horizontal dimensions of the face 8 allows reliable contact with a golf ball for the full range of impact offset-errors encountered during normal play. For the purpose of further description of the present invention, reference will be made specifically to The main effect required of the impact is to launch the golf ball 13 with linear velocity aligned with the line of intended putt. The linear velocity is proportional to the hit velocity of the head 1, and the ball 13 would attain a maximum value of linear velocity v_{c }with no spin when the impact-face 8 is square to the direction of movement of the head 1 and the center of mass 15, the center of mass of the ball 13 and the point of impact are co-linear. However, when the normal to the impact-face 8 at the point of impact is above the center of mass 15 as in the case represented in It has been found that the position of the borehole 16 (
Further, if the shaft has radius r, wall thickness t and that r>>t (which is usually the case), then: Equations (1) to (3) also show that non-standard shafts could reduce rotational stiffness. For t<<r, the second moment of area about a bending axis increases as r^{3 }but the cross-sectional area increases as r, so increasing r and t, while decreasing E (for example, replacing steel with an engineering plastic) can provide a shaft with the same flexural stiffness as a standard shaft but much less axial stiffness. This reduces rotational stiffness about the heel-toe axis when it is necessary to have d_{1}>r. In practice, shaft diameters of 10 to 20 millimeters or greater (compared to 9.4 millimeters in a standard shaft) can be provided using low modulus material to provide a shaft with ample strength and durability, but with much reduced stiffness. With steel putter-shafts the radius and/or wall thickness of the shaft tip can be reduced. This allows for the fact that standard steel putter-shafts with diameter 9.4 millimeters have much higher strength and stiffness than necessary, with greater flexural stiffness than driver shafts, which are subjected to much higher stress. Thus, steel or other alloy putter-shafts with small, non-standard shaft-tip diameters are usefully employed. Further reduction in torque stiffness can be provided by arranging that the end of the shaft attaches to the putter-head at, or more preferably below, the heel-toe axis through the putter-head center of mass. This is illustrated by reference to
The above qualitative analysis with reference to (a) to (c) of In practice, attaching the end of the shaft below the heel-toe axis is difficult with conventional means; typically an epoxy adhesive joint is used with the bonded section of the shaft extends 10 millimeters depth or more into the putter-head. Thus, implementation will require development of new attachment means where only a few millimeters of the shaft end is needed to make a reliable join. It is found that minimising the torque stiffness also advantageously modifies the acoustic and vibration characteristics at impact. In this respect, it is also advantageous to arrange that the borehole or other means of shaft attachment is not only close to the heel-toe axis as described above, but positioned at, or close to, the center of mass. Other means of reducing the torque stiffness due to shaft attachment can be provided, including special low-stiffness shaft or shaft-coupling arrangements. A hosel extension or neck can be provided between the putter-head and the shaft-attachment point with small and elongate section to reduce torque stiffness about the heel-toe axis but maintain adequate strength and robustness. Traditionally, an adhesive is used to bond the shaft end into the borehole of the putter-head, so the resilience and thickness of the adhesive material can be designed to allow higher compliance, without compromising the stability and ruggedness of the bond. It is established teaching that the head of a golf club (including that of a putter) behaves as a free body during impact. That is, during the very brief time of contact (less than one millisecond), the shaft has negligible influence on the outcome of the impact; see for example: Cochran, A. and Stobbs, J. 1968. Search for the Perfect Swing, Chicago: Triumph Books, p. 147. It is thus known that the outcome of a golf shot (including a putt) can be analysed as a case of eccentric, oblique impact in a two-body system comprising the ball and club-head only. From this, exact equations can be derived that predict the launch velocity and spin components of a golf ball. These equations give accurate prediction of many aspects of club-on-ball collision in golf. For convenience, we refer to these equations as ‘basic impact equations’. For example, it is well known that the so-called ‘sweet spot’ of a putter-head is normally mid-way between the heel and toe and corresponds almost exactly to the position of the center of mass along the heel-toe axis. At the sweet spot, the ball launch velocity as a function of putter-head swing speed reaches a maximum, no head rotation from impact occurs and the contact is ‘solid’ with minimum vibration and sound—hence the name ‘sweet spot’. This result is exactly as expected from the basic impact equations. However, applicant's measurements of putter-head impact characteristics show that whereas the basic impact equations accurately predict dynamic behaviour for lateral eccentric impacts that cause rotation about the putter-head vertical axis, the prediction is inaccurate for vertical eccentric impact. In vertical eccentric impact (above or below the sweet spot), the putter-head tends to rotate about the horizontal heel-toe axis. In this mode, the putter shaft presents maximum resistance to movement but the putter-head moment of inertia is a minimum (being especially small in putter-heads according to the present invention). Thus, the model of the putter-head as a free body is least representative for this mode. This gives rise to significant discrepancies between measured performance and performance predicted by the basic impact equations. By contrast, in lateral eccentric impact, the shaft presents much lower resistance to movement about the putter-head vertical axis and, moreover, the moment of inertia about this axis is almost invariably a maximum (due to the practice of ‘heel-toe weighting’ to minimise putter-head rotation in this mode). Thus, the basic impact equations provide a much more accurate model for lateral eccentric impacts and also accurately predict the effect of direct oblique impact (as distinct from eccentric impact) since no club-head rotation at impact is involved and the shaft constraining forces are negligible. A theoretical treatment of the ball-on-putter-head impact taking account of the shaft constraining forces would be very difficult and complex. In the present context therefore, the putter-head is considered as a free, rigid body detached from its shaft during impact, to which the basic impact equations are applied to predict performance. From this the maximum gear-effect attainable is calculated assuming no shaft constraining forces, and the result is then qualified to take account of the possible effect of torque stiffness during impact due to the shaft. It is to be appreciated that any practical putter-head with suitable shaft attachment means can provide substantially all the available gear-effect performance predicted by the basic impact equations provided the shaft is sufficiently compliant. New shaft types for putters may be produced to satisfy this special requirement, but other factors, such as design aesthetics or user-preferred shaft type and set-up, may dictate the overall design so that all the available gear-effect performance is not utilised as a compromise between desired topspin performance and other factors. The variables (including fixed golf-ball parameters) used in the basic impact equations comprise the mass and inertia parameters of the golf ball and putter-head, the ball radius, and the geometry or shape parameters of the putter impact-face. For the putter-head, the variables (and preferred units assumed herein) are:
The gear-effect real ised with a putter-head is dependent on the condition that the line of impact (i.e. the line normal to the impact surfaces at the point of impact) is offset from the center of mass of the head. It follows that the condition for gear-effect in the present invention is also dependent on the impact-face loft angle at the point of impact. The offset h between the line of impact and the center of mass, is:
To impart topspin on average rather than backspin, the average value of h must be positive. A golfer of average skill can execute putts with average impact heights of 12 millimeters or above, so the condition for h positive (on average) can be met if h_{c }is less than:
Loft angles in puffers are seldom greater than 5 degrees and more usually 3 degrees or less, so it can be seen that the third term on the right-hand side of equation (4), is only significant if p is large as in accordance with the present invention. The combination of high gear-effect and its sensitivity to loft angle allows useful modification of loft angle to enhance performance in putters of the present invention. To a first approximation, the basic impact equations predict that the topspin initially increases linearly with both h and p and increases as the inverse of the putter-head moment of inertia (with radius of gyration K) about the horizontal heel-toe axis through the putter-head center of mass. The equations also show that as p is greatly increased the spin rate (for a given h) reaches a maximum and thereafter reduces, but this only occurs with unusually large p, so for most practical putter-heads it is safe to assume that increasing p increases the available topspin performance. Where the shaft attachment is positioned close to the impact-face of a putter, the effective value of p is likely to be less than its true value. A direct result of increasing p is that sidespin from lateral eccentric impacts also increases. That is, when the impact point is offset from the sweet spot towards the heel or toe, an increased value of p gives rise to increased sidespin (all other factors being equal). The imparted sidespin is believed to have negligible influence on the direction of the putt (see Cochran, A. and Stobbs, J. 1968. Search for the Perfect Swing, Chicago: Triumph Books, p. 131), but the basic impact equations predict that a sideways component of launch velocity proportional to the sidespin magnitude is generated, which give rise to directional errors. This result has been verified by measurement and shown to be in close agreement with errors predicted by the basic impact equations. In this regard, it is an aim of the present invention to limit directional errors by providing sufficiently large values of l in putter-heads according to the invention, so that launch angle errors due to lateral impact offsets of ±12.7 millimeters (i.e. ±0.5 inch) are not more than ±1.6 degrees. In golfing terms, this corresponds to just sinking a six-foot putt providing all other aspects of the putt are perfect. A closely equivalent criterion (which derives from the basic impact equations) is that the ratio p/l should be less than 0.18. Since topspin increases as the inverse of moment of inertia about the heel-toe axis, limiting this moment of inertia is another aim of the present invention. This involves selecting the overall putter-head mass and controlling the distribution of this mass about the heel-toe axis. Putter-heads are commonly made with mass in the range 0.25 kilogrammes to 0.45 kilogrammes but a narrower range of about 0.3 to 0.35 kilogrammes is preferred by many manufacturers. We thus see that the mass of a putter-head is traditionally kept within fairly narrow limits, presumably to reflect players' preferences. Therefore, control of the moment of inertia plays an important part in the design of putter-heads according to the present invention. Moment of inertia is proportional to the square of radius of gyration, so small changes in K can significantly alter the topspin performance. The applicant has found that the position of the shaft attachment can alter the effective value of K and this effect is seen as the most significant in regards to the problem of torque stiffness due to shaft attachment. It has been proved experimentally that with both d_{1 }(the horizontal offset between the shaft attachment axis and the heel-toe axis) and d_{2 }(the vertical offset between the shaft point of attachment and the heel-toe axis) nearly zero, the performance of prototype putters according to the invention closely agrees with the performance predicted by basic impact equations, whereas increasing either d_{1 }or d_{2 }reduces the imparted topspin and also reduces the variation in launch velocity as a function of impact height. Thus, there is empirical evidence that if either d_{1 }or d_{2 }is greater than zero the effective radius of gyration K_{e }is greater than the basic putter-head radius of gyration K (both measured about the heel-toe axis through the putter-head center of mass). It follows that the ratios d_{1}/K and d_{2}/K are important design factors. It is considered that d_{1}/K should be less than +1.0 or more preferably, less than +0.33 and similarly, d_{2}/K should be less than +1.0 or more preferably, less than +0.33. However other design considerations may determine that one or both of these ratios is greater than +1.0. A further advantage of positioning the shaft coupling close to the center of mass is that shaft vibrations due to eccentric impact are minimised. In this respect, it is advantageous that the axis of the shaft attachment means passes close to the putter-head center of mass (as distinct from the heel-toe axis through this center). Some experimental evidence indicates that this arrangement is also best for ensuring that the assembled putter advantageously behaves most closely to the model predicted by the basic impact equations. It is thus preferable that the axis of the shaft attachment means is offset by less than K millimeters, or more preferably not more than the putter-shaft radius r millimeters, from the putter-head center of mass, measured in any direction. A putter-head in accordance with the invention, of ‘mallet’ style as distinct from the ‘blade’ style of the putter-head of Referring to For cosmetic purposes, the casing 31 may be transparent or translucent and may be colour tinted or clear, with the element 32 visible through the casing. In this case, the element 32 may have a legend, emblem or other information printed, engraved or cast into it and visible inside the putter-head 30 through the casing 31; this allows the bottom surface or sole of the putter-head to contain such information but still be perfectly smooth. The attachment socket 35 for the putter-shaft is located vertically above the heel-toe axis through the center of mass of the head 30. More particularly, the socket 35 is angled so that its axis, and accordingly the longitudinal axis of the shaft, extends less than K millimeters from the center of mass. The impact-face of the head 30 is provided by an element 36 that is secured to an upstanding flange 37 of the casing 31; alternatively, the element 36 may be an integral part of the flange 37. As shown in The boundary between surfaces 38 and 39 is at height h_{α }millimeters. The lofted surface 38 slightly reduces topspin for high values of impact-height h_{i }but improves the achieved length of putt by shifting the line of impact back towards the center of mass at height h_{c}. Here, abundance of topspin is exchanged for slightly less topspin but improved distance control. Also, the progressively de-lofted surface 39 extends topspin and putt length at low values of impact-height h_{i}. The line of impact is raised relative to the center of mass and negative oblique impact is introduced; both assist topspin and extend putt distance with low impact-height h_{i}, the compromise being that negative loft is introduced. Negative loft occurs only at the lower section 39 of the impact-face. This is not disadvantageous in practice since putting styles with high loft at impact tend to result in impacts with low height h_{i }so that negative loft is cancelled by the orientation of the putter-head at impact. Conversely, putting styles with low loft at impact tend to result in impacts with high values of height h_{i }where the positive loft of the putter and/or high topspin helps to lift the ball at impact. The degree of imparted topspin (or backspin) from a putter impact is conveniently quantified in terms of the ratio (namely, S percent) of the ball peripheral velocity due to spin to its linear velocity (that is, the translational velocity of the ball center). From the basic impact equations, S can be approximated as two independent terms: one linearly proportional to h (defined in equation (4) above) and the other linearly proportional to the obliqueness angle of the impact. The obliqueness angle is dependent on the putter-face loft angle at impact (α_{i}) and also on the putting style (namely, whether the style is ‘pendulum swing style’ or swung with the putter shaft tilted forwards or backwards at impact). In putters according to the invention, the component of S due to oblique impact is very small compared to the gear-effect (eccentric impact) component. The function dS/dh is the rate of change of imparted spin as a function of h and is very nearly constant for any given putter-head with given values of M, K and p, and provides an important measure of putter-head performance.
Curves 1 to 6 show that for a typical range of putter-head mass, the spin rate decreases with K and M and is particularly sensitive to K. The moment of inertia of the putter-head about the heel-toe axis through the center of mass is equal to (M×K^{2}) and this is evaluated in the fourth column of Table 1. From this, it can be seen that spin rate is approximately inversely proportional to the heel-toe axis moment of inertia. Also, for any given value of heel-toe axis moment of inertia, a value of p equal to about [8.9×K×M^{1/2}] gives the maximum value for the constant dS/dh. However, these values of p are very difficult to implement. For example, with K equal to 14 millimeters, a putter-head of 0.3 kilogrammes requires a value for p of about 70 millimeters, which results in a very large and cumbersome putter-head. On the other hand, small values of K such as 8 millimeters can only be achieved in putter-heads of normal length and mass by using very expensive materials such as tungsten alloys, combined with low density, high modulus composites. Even then, attaining maximum dS/dh is very difficult. For convenience, a function G which provides a measure of the dS/dh characteristic of a putter-head is defined as:
In prior art, typical values for K are 11 to 12 for blade style putters, increasing to 14 to 17 for mallet styles, with corresponding p values of about 8 to 10 (blade style) increasing to 14 to 18 (mallet style). Thus, the value of G in the prior art is normally greater or much greater than 350. To allow enhanced topspin in putter-heads according to the invention, the value of G should be less than 350. For preference G should be less than 250, or more preferably less than 170. It has been found that a putter-head design with exceptionally high topspin can result in severe loss of linear velocity and consequently, loss of putt distance. It is thus desirable to calculate the variation in putt length as a function of launch velocity and imparted topspin variations, and use this information to modify, if necessary, putter-head parameters so as to obtain satisfactory putt distance performance. The theory of spherically symmetrical balls sliding and/or rolling on a flat uniform surface is well documented. Thus, exact equations can be derived predicting the initial linear deceleration and accompanying rotational acceleration due to sliding friction and, once pure rolling motion is achieved, linear deceleration by rolling friction that eventually slows the ball to standstill. In
A standard golf ball radius is only 21.3 millimeters, so a possible putter impact height of 20 millimeters allows a very small clearance above ground to avoid dragging effects of the turf. With impact height of 2 millimeters or less, the ball is struck very near the bottom lip of the impact-face, where impact consistency becomes unreliable and launch elevation becomes increasingly negative. There is thus a possible range of about 2 to 20 millimeters, but in practice impact height will rarely exceed limits of between 5 to 17 millimeters and will average around 10 to 12 millimeters. Three curves A, B and C superimposed on the curves 1 to 6 in
Head 1 is based on the putter-head of In Traces C and D of Traces E and F of Because imparted topspin in the present invention relies on impacts being off-center from the ‘sweet spot’, levels of head-vibration can be greater than that obtaining in a conventional putter where sweet-spot impact is expected. The known method of reducing such vibration is to clad the metal or other low-loss parts with high-loss materials. Advantageously, the major part of the putter-head according to the invention can be made from a high-loss material such as a polymer or composite (Ashby, M. A. 1992. Materials Selection in Mechanical Design, 2^{nd }ed., Oxford: Butterworth-Heinemann, pp. 46–48). If necessary, cavities or recesses in the putter-head can be provided and filled with high loss materials, provided these do not reduce the overall rigidity of the putter-head. Other methods for low-frequency vibration control include filling the shaft with vibration dampening material such as granular material sold under the Trade Mark LODENGRAF. Referring now to The greater part of the head-mass is provided by the rear-body section 41 which preferably extends, heel to toe, at least 120 millimeters or significantly more (so as to be longer than an average putter-head). This helps to reduce the moment of inertia about the heel-toe axis and advantageously increases the moment of inertia about the central vertical axis. The provision of a chevron-shaped cut-away 42 between the rear-body section 41 and the front flange 43 with its impact-face 44, further reduces the heel-toe moment of inertia. The cut-away 42 also provides an alignment aid when addressing a ball during play. Small differences in alignment relative to the intended line of putt are shown up by the obtuse angle 45. In an alternative arrangement, the cut-away 42 is replaced by a thin plate section and the chevron-shaped feature is highlighted with a contrasting-colour paint. In another arrangement, a bridge part can be provided across the center, so dividing the cut-away into two symmetrical apertures. In this embodiment of the invention, the putter-shaft (not shown) is attached at or close to the center of mass of the putter-head by a re-entrant ‘over-hosel’ arrangement (best seen in The graph of The solid trace and dotted trace in It can be seen that the roll feature, which progressively reduces loft angle on the lower half of the impact face, significantly improves the putt length consistency. Without the reduction of loft angle in the lower part of the impact face, the putt length versus impact height characteristic is severely variable as shown in the dashed trace of In an alternative embodiment, the reduction of loft in the lower half may be abrupt rather than progressive. That is, the impact face may have two or more flat surfaces with upper surfaces more lofted than lower surfaces. This is particularly suited to soft impact faces. Note that the maximum putt length is usually achieved slightly above the ‘sweet spot’ of a putter (where the imparted linear velocity is maximum) because of the positive slope of the spin versus impact height characteristic. However, assuming that the maximum putt length occurs at the sweet spot (when h=0) provides a fairly accurate means of calculating the performance of putter-heads according to the invention. Using this approximation, we define a relative putt length LR as the putt length at any impact height relative to the putt length at the sweet spot as follows:
A preferred method of designing putter-heads according to the invention is to use Equation (6) to evaluate the putt lengths LR_{5 }and LR_{17 }at impact heights of 5 and 17 millimeters respectively. Preferably, both LR_{5 }and LR_{17 }should be greater than 0.95 but more preferably greater than 0.98 (such as in Head 3 above). To evaluate Equation (6) at impact height of 5 millimeters, the values of V and S are found from the following equations: The principal heel-toe axis is the horizontal axis parallel to the impact face that passes through the putter-head center of mass. To measure rotational compliance about this axis, the head may be held in a freely rotating chuck or similar device with the chuck axis and the principal heel-toe axis co-linear and the shaft rigidly clamped close to the point where it attaches to the putter-head. The chuck is then rotated through a small angle (e.g. 1 to 2 degrees) with a torque wrench and the compliance in degrees per Newton-meter measured. High rotational compliance about principal heel-toe axis can be arranged by positioning the The putt length plots shown in The putt length plot for Head 3 (solid trace in Referring again to Putters according to the invention preferably conform to The Rules of Golf. In this respect, the following must apply:
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